NUMERICAL CONTROL DEFINITION
Programmable automation in which the mechanical actions of a ‘machine tool’ are controlled by a program containing coded alphanumeric data that represents relative positions between a work head (e.g., cutting tool) and a work part THREE BASIC COMPONENTS OF NC
1. INPUT MEDIUM 2. MACHINE CONTROL UNIT 3. MACHINE TOOL
Principle
A system in which actions are controlled by direct insertion of numerical data.
COMPUTER NUMERICAL CONTROL (CNC)
• Storage of more than one part program • Various forms of program input • Program editing at the machine tool • Fixed cycles and programming subroutines • Interpolation • Acceleration and deceleration computations • Communications interface • Diagnostics DIRECT NUMERICAL CONTROL (DNC) NC Position Movement Incremental > Taking the “Last” position as the zero position.
Absolute > Locations on Part – Fixed Reference Frame with Home position for reference. REFERENCE POSTIONING SYSTEMS
INCREMENTAL ABSOLUTE Degree of Motion Control
Point-to-Point (PTP) > Good for holes & slots > Position tool over point.
Contouring > Complex curved surfaces > Computers needed for complex calculations > Motion control to motors: varying voltages to DC servo motors. Motion Control Systems Point-to-Point systems • Also called position systems • System moves to a location and performs an operation at that location (e.g., drilling) • Also applicable in robotics
Continuous path systems • Also called contouring systems in machining • System performs an operation during movement (e.g., milling and turning) PTP CONTOURING Interpolation Methods 1. Linear interpolation Straight line between two points in space 2. Circular interpolation Circular arc defined by starting point, end point, center or radius, and direction 3. Helical interpolation Circular plus linear motion 4. Parabolic and cubic interpolation Free form curves using higher order equations NC Part Programming
Bit - 0 or 1 = absence or presence of hole in the tape Character - row of bits across the tape Word - sequence of characters (e.g., y-axis position) Block - collection of words to form one complete instruction Part program - sequence of instructions (blocks) Block Format Organization of words within a block in NC part program Also known as tape format because the original formats were designed for punched tape Word address format - used on all modern CNC controllers Uses a letter prefix to identify each type of word Spaces to separate words within the block Allows any order of words in a block Words can be omitted if their values do not change from the previous block N - sequence number prefix G - preparatory words Example: G00 = PTP rapid traverse move X, Y, Z - prefixes for x, y, and z-axes F - feed rate prefix S - spindle speed T - tool selection M - miscellaneous command Example: M07 = turn cutting fluid on Example: Word Address Format
N001 G00 X07000 Y03000 M03 N002 Y06000
Advantages of NC
Flexibility Capability for Complex Work-pieces Manage Large Work-pieces Reduced Jig & Fixture Cost Higher Quality Direct Numerical Control
Advantages: > Library of programs > Instant modifications > Links with CAD > Increase Information Response > Instant Reports Computer Numerical Control (CNC)
Advantages: > CRT allows review/editing > Pre-check/simulation > Interface allows more capability > Accurate positioning > More functions NUMERICAL CONTROL PROGRAMMING
COMPUTER NUMERICAL CONTROL PROGRAMMING
•X axis controls the table movement left or right.
• Y axis controls the table movement toward or away from the column.
• Z axis controls the vertical (up or down) movement of the knee or spindle. Positioning Reference Point Systems - INCREMENTAL
Positioning Reference Point Systems - ABSOLUTE CNC Positioning Systems - Point-to-Point or Positioning Types of contour machining (A) Simple contour; (B) complex contour Interpolation
Method by which contouring machine tools move from one programmed point to the next is called interpolation 1. Linear 2. Circular 3. Helical 4. Parabolic 5. cubic Linear Interpolation Circular Interpolation PROGRAMMING CODES
G Codes and M Codes G CODES - preparatory functions
M CODES - miscellaneous functions
F – Feed S – Speed D – Cutter diameter offset T – Tool Number Block of Information
complete block of information consists of five words • N001 represents the sequence number of the operation. • G01 represents linear interpolation
• X12345 will move the table 1.2345 in. in a positive direction along the X axis.
• Y06789 will move the table 0.6789 in. along the Y axis
• M03 Spindle on CW. Graving Tool
TOOL PATH FOR MACHINING THE COMPONENT
DRILLING EXAMPLE FOR CANNED CYCLES SIMPLIFIED PART PROGRAM USING CANNED CYCLED ‘G’ Code STANDARD CANNED CYCLE MOTIONS CUTTER RADIUS COMPENSATION 62 63 64 PROGRAM STRUCTURE
65 N5 G X Y Z S M F T D LF
G Code Spindle Tool Fn Speed Coordinate Values End Sequenc of e No. Block Block No. M Code Feed
Tool offset fn
66 G Codes – Preparatory functions
67 68 69 70 M00 – PROGRAMME STOP UNCONDITIONAL M Codes – M01 – PROGRAMME STOP OPTIONAL Miscellaneous Functions M02 – END OF PROGRAMME M03 – SPINDLE ROTATION CW M04 – SPINDLE ROTATION CCW M05 – SPINDLE STOP M08/09 – COOLANT ON/OFF M17 – SUBROUTINE END M19 – SPINDLE ORIENTATION M30 – END OF PROGRAMME
71 72
74 Absolute/incremental dimensioning (G90, G91)
75 Inch/metric input (G70/G71) or (G20/G21)
76 N10 T1 M6 ; Loading of tool T1
77 N20 G96 S100 LIMS=2500 ;
Explanation Constant cutting rate = 100 m/min, max. speed 2,500 rpm
78 Settable zero offset – G54
79 EXTENDED WORD ADDRESSES
ANG = to assign exterior angle AR = apex angle AC = absolute coordinate AP = polar angle CIP = circular interpolation CT = circular tangent CHF = chamfer (general use) CHR = chamfer (base distance) CR = radius for circular interpolation CYCLE = machining cycle GOTO = jump function IC = incremental coordinate LIMS = upper spindle speed limitation MSG() = message (max 65 characters) RND = rounding RP = polar radius SPOS = spindle position in degrees 80 TURNING MILLING
81 EXAMPLE
82 83 84 85 86 Workshop on CNC programming 87 DME, RSET